1. Field of the Invention
The present invention relates to optical communication networks in general and, more particularly, to bidirectional optical communication networks in which two wavelength division multiplexed optical communication signals propagate in opposite directions on a bidirectional optical waveguide. Following a waveguide failure (e.g., fiber cut) or equipment failure, optical traffic is successful re-routed in order to avoid an interruption in communication services.
2. Description of the Related Art
As the need for communication signal bandwidth increases, wavelength division multiplexing (WDM) has progressively gained popularity for multiplying the transmission capacity of a single optical fiber. A review of optical networks, including WDM networks, can be found in Ramaswami et al., Optical Networks: A Practical Perspective (Morgan Kaufman, .COPYRGT. 1998), the disclosure of which is incorporated herein by reference. Typically, wavelength division multiplexed optical communication systems have been designed and deployed in the long-haul, interexchange carrier realm. In these long-haul optical systems, a wavelength division multiplexed optical communication signal comprising plural optical channels at different wavelengths travels in a single direction on a single fiber (unidirectional transmission). Because the communication traffic in such systems commonly travels many hundreds of kilometers, the need for add-drop multiplexing of individual channels is infrequent (if at all), occurring at widely-spaced add-drop nodes.
Although the optical infrastructure of long-haul WDM optical systems can accommodate future traffic needs created by increased demand from traditional and multimedia Internet services, this traffic must first be collected and distributed by local networks. Currently, such local networks are structured to carry a single wavelength, time-division multiplexed (TDM) optical signal along a fiber network organized into various ring structures. To route the various components of the TDM signal, numerous electronic add-drop multiplexers are positioned along the fiber network. At each add-drop location, the entire optical signal is converted into an electrical signal; the portions of the electrical signal which are destined for that add-drop point are routed accordingly. The remaining portions of the electrical signal are converted back to a new TDM optical signal and are output through the electronic add-drop multiplexer. Thus, before a user can access the bandwidth-rich WDM long-haul transport networks, he must first pass through the bottleneck of the local networks.
Although unidirectional WDM optical systems are suitable for conventional long-haul interexchange carrier markets(e.g., "point-to-point" optical systems), metropolitan (local) communications systems require extensive routing and switching of traffic among various nodes positioned within optical fiber rings. Further, in order to maximize the effectiveness of wavelength division multiplexing in these local areas, it would be useful to implement bidirectional WDM optical systems, e.g., to enhance network design flexibility. In a bidirectional WDM system counter-propagating WDM optical signals, each of which carry a number of optical channels, are carried on the same waveguiding medium, such as a single optical fiber. Implementation of a bidirectional system requires several considerations not present in the conventional unidirectional optical systems.
One such consideration is the ability to switch communication traffic from a "work" path to a "protect" path in the event that there is a disruption in the waveguiding medium (e.g., a fiber cut) or there is an equipment failure at any point within the optical system. In conventional, unidirectional optical systems, optical traffic is frequently routed to another optical waveguide or another optical ring. Such techniques are depicted in U.S. Pat. Nos. 5,982,517 and 5,327,275. Although these systems permit continuation of optical traffic in the event of a fiber cut, they require the presence of an additional optical path, such as a spare optical fiber; such extra capacity is often in short supply in crowded metropolitan regions. Further, since "protect" optical fibers often are damaged during disruption of the "work" fiber, such protection switching may not be available.
Unique issues are presented in bidirectional WDM optical communication systems since both east-west and west-east WDM optical signals propagate along a single optical waveguide. Consequently, if the waveguide is interrupted, optical signals traveling in each direction must be re-routed. Complexity is increased if the system features "wavelength re-use," i.e., when a wavelength used to carry traffic along one span between two given optical nodes is later employed to carry optical traffic between two different optical nodes. For bidirectional optical systems featuring wavelength re-use, care must be taken that optical traffic routed to a protect path does not interfere with work traffic traversing the same optical span.
Thus, there is a need in the art for improved protection switching systems for wavelength division multiplexed optical communication networks, in particular, bidirectional WDM optical networks. Such improved protection switching could be used to implement wavelength division multiplexing in fiber-constrained metropolitan networks.